Computers in platforms ranging from individual workstations to supercomputers generally use densely packaged or stacked electronic modules to enable higher operating speeds through reduced signal latency. These modules perform the various computational and control functions. There are many types of modules such as processor element modules, power modules, control modules, clock modules, daughter card modules, and others, all of which include numerous forms, types and sizes of integrated circuits and electronic components. Most of these modules are more densely packaged, process information faster, and produce more waste heat than their predecessors. The tight packaging leaves less room for providing paths to dissipate the increased waste heat.
Proper cooling is essential in today's computers; excess heat can build up, causing malfunction and failure of integrated circuits and other electrical components. The importance of effective and efficient cooling for each application grows as circuit board assemblies and modules become smaller and more densely packed with ever faster and smaller components. Hence the need for an improved method and apparatus to dissipate the heat.
One way of cooling such components includes incorporating one or more cooling or cold plates within the module. A typical cold plate may either be liquid cooled or air cooled depending on the cooling system and necessary cooling capacity required for the computer into which the plates are installed. The cold plate typically is custom designed to meet the needs of a particular application. A typical cold plate is mounted adjacent heat-producing objects and either directly or indirectly connected via a thermally conductive path to the objects. The cold plate usually has one or more paths or flow channels for air or a liquid coolant to flow through the plate and carry away heat transferred from the objects to the plate. Cold plates may further communicate with a larger cooling system external from the computer or may simply dissipate the heat to the atmosphere.
Another means used for managing waste heat is the use of relatively large discrete heat sinks strategically placed on a printed circuit board adjacent or on top of high power electronic components. A typical heat sink is produced from a material such as aluminum and may include projecting fins or pins for added surface area. The heat sink is in thermal conductive contact with the heat-producing device and absorbs the heat for dissipation to the atmosphere or to a remote reservoir. The size of a discrete heat sink depends on the amount of heat generated by the device. Discrete heat sinks add to the pitch or height of PCB's causing increased electrical signal paths. Discrete heat sinks also require paying attention to component placement on the PCB causing thermal shadowing by blocking air flow over the PCB. Component placement limitations in turn affect electrical design versatility and the capability of the design to accept product upgrades and enhancements.
Another type of liquid cooling system is an immersion system wherein the heat-producing components are submerged in a liquid coolant. This type of system has excellent cooling capacity but is difficult and expensive to maintain. An immersion system is often limited in its application (primarily due to cost and weight) and is particularly unsuitable for workstation systems. Immersion cooling is often unacceptable as all components' materials must be compatible with the dielectric fluid.
Direct transfer to air cooling systems have the advantages of using no hidden secondary cooling/transfer system and have low energy cost limited typically to only a fan of some type to produce a constant air flow across the computer. However, direct air-cooled systems have limited cooling capacity. Liquid-cooled systems have the advantage of providing a high heat capacity resulting in excellent cooling characteristics. A disadvantage of liquid cooling systems is that a remote support system for the coolant is typically required and must be accommodated in the overall computer packaging scheme. Commonly, heat transfer systems require rotating equipment (i.e., fan or pump) to move heat through a distance. The heat pipe system is "self-pumping" in that heat is driven to the condenser section by vapor pressure and returned liquid is brought back to the evaporator by capillary force. Other disadvantages are that liquid cooled systems are expensive to acquire, install, run and maintain and have not achieved acceptability in size. A further disadvantage is that they produce thermal gradients along the coolant flow path resulting in varying component temperatures which in turn results in poor and inconsistent component performance.
Another cooling method which is presently employed utilizes intricate mechanical spreading systems. This type of system is only suitable for high-end computer systems because of high manufacturing costs, complexity, high volume and energy consumption and limited small scale thermal capabilities. Material advances have provided wafer substrate materials and thermal interfaces which efficiently spread or transfer heat to a desired region of a spreading system but the costs involved are excessive when considering the workstation environment.
Commonly available air-cooled heat sinks dissipate heat via a conduction path to an extended surface area. The fin area is limited in size due to conduction loss and associated decreases in efficiency as the distance from the base area increases. The heat pipe thermal management system presented permits almost unlimited growth in this transfer area as it expands the base area of the condenser region (and avoids increasing board-board pitch).
In today's competitive market, the computer industry is driven to reduce the size and increase the speed of computers and components, reduce the number of parts and part variations, reduce manufacturing and assembly complexity, and reduce manufacturing, assembly and part costs. One way the industry has used to achieve these goals is to attempt to produce standardized, interchangeable components and modular assemblies to eliminate the prior practice of using custom-designed components. Single-chip packages are being replaced by multi-chip packages which result in higher-powered, smaller, faster and hotter electronic components.